The predictive value of a bronchial challenge with histamine was determined in a prospective survey on a population with a high prevalence of asthma (0.62). Without knowledge of the bronchial responsiveness 133 patients were classified as asthmatics (83) or non-asthmatics (50) according to variation in peak expiratory flow rate and medical history. Response to challenge was determined by the interrupter method, and the concentration of histamine inducing a 40% increase in resistance to breathing (PC40-Rt) was calculated from the log dose response curve. When defining a positive test as a test giving PC40-Rt-values below 2.00 mg/ml, the predictive value of a positive test was 0.75 and the predictive value of a negative test was 0.72. By decreasing the limit for a positive test to 0.25 mg/ml the corresponding predictive value was increased to 0.91. When further increasing the limit to 4.00 mg/ml the predictive value of a negative test in the diagnosis of asthma was increased to 0.81. The interrupter technique is suitable for diagnostic purposes in the detection and exclusion of bronchial asthma.